Recently, several authors proposed that the availability of intermediate hosts (IH) for definitive hosts (DH) may contribute to determine the dynamics and evolutionary ecology of parasites with facultative complex life cycles. The protozoa Toxoplasma gondii may be transmitted either through complex life cycle (CLC) or through simple life cycle (SLC) and is present in contrasting host density environments. We tested the hypothesis that the relative contributions of CLC and SLC along an urban-rural gradient depend on the IH supply. We built and analyzed a deterministic model of T. gondii transmission cycle. SLC relative contribution is important only in urban-type environments, i.e., with low predation rate on IH. In contrast, the parasite is predominantly transmitted through CLC in suburban and rural environments. The association of the two cycles enables the parasite to spread in situations of low IH availability and low DH population size for which each cycle alone is insufficient.